A few minor UI comments. + Failing to select an amount to send should raise a flag. + Entering a negative amount to send should raise a flag. + After viewing

A few minor UI comments. + Failing to select an amount to send should raise a flag. + Entering a negative amount to send should raise a flag. + After viewing a result, if I hit `back` on my browser (say, to compare results for a different transfer amount), it resets the currencies to their defaults (for me, U.S. and Argentina). It would be nicer to remember my selections.